A heat shrinkable tube made from a polyolefin containing a polyethylene is normally produced by the following method.
That is, first, a polyolefin is molded into a tube by means of extrusion, and crosslinking is carried out by means of electron beam irradiation, and then, the tube is heated to a temperature equal to or higher than the softening point of polyolefin. The tube is expanded in this condition and cooled in this expanded condition.
The process in which a tube is heated to a temperature equal to or higher than the softening point of the polyolefin, expanded in the heated condition, and then cooled in the expanded condition is referred to as an expansion process, and in this expansion process, poor concentricity in which the tube thickness is not uniform may occur.
In a case of a thick tube or a tube using a polyolefin with a low degree of crystallinity, poor concentricity does not easily occur, however; in a case where a polyethylene with a high degree of crystallinity is used to form a thin tube, poor concentricity easily occurs.
In a case of a heat shrinkable tube for insulating cover of a capacitor, improvement in wear resistance and improvement in insertability of a covering object into the tube are desirable. In order to improve these, use of a medium-or high-density polyethylene with a high degree of crystallinity, and the like, is preferable.
However, in this case, poor concentricity easily occurs as described above, and the permissible poorest concentricity is considered as follows.
For example, when a marking is printed with a marking roll pressed against a flattened tubing, the tubing must be sufficiently flat so that the marking may be printed well. It is known that such printing can be carried out without problems if the concentricity is 25% or less, where the concentricity=(maximum wall thickness−minimum wall thickness)+maximum wall thickness×100.
Furthermore, in a case of a heat shrinkable tube for insulating cover on a capacitor, it is required to reduce the wall thickness; however, the smaller the wall thickness, the more easily thermal deterioration occurs. Therefore, it is difficult to satisfy a heat aging test, copper stability test, and heat shock test that are necessary to obtain UL certification.